(1) Field of the Invention
This invention relates to high strength austenitic steel having excellent cold work hardenability, i.e., capable of developing a 0.2% proof stress of 130 kgf/mm.sup.2 or higher when hardened through cold working. More specifically, it relates to high strength austenite steel having excellent cold work hardenability and suitable as a material for generator retaining rings and the like.
(2) Description of the Prior Art
It is generally required for materials adapted for fabrication of generator retaining rings that they be non-magnetic to avoid drops in the efficiency of power generation and, in addition, that they meet ever-increasing demands for strength to cope with the trend of capacity increase of power generators. Cold worked 0.5C-18Mn-5Cr steel is presently used to fabricate such retaining rings.
Although 0.5C-18Mn-5Cr steel has a high degree of strength, it still involves a problem in that it develops stress corrosion cracking through its repeated use over a long period of time. It has been confirmed through experiments that its resistance to stress corrosion cracking is considerably lowered by moisture deposition although the mechanism of this stress corrosion cracking has not yet been completely elucidated.
As a potential solution to the problem of stress corrosion cracking of retaining rings, high Cr steel (Cr content: 13 wt.% or more) having excellent resistance to general corrosion has been attracting considerable attention. However, such high Cr steel is accompanied by a drawback in that its C-content must be maintained at a lower level to avoid the formation of Cr carbides because any abundant Cr-content promotes the formation of Cr carbides and the resultant steel is reduced in its resistance to general corrosion to the same level as low Cr steel.
A lower C-content makes it difficult to highly strengthen steel through its cold working at 350.degree. C. or lower. Thus, a very high degree of processing and working is indispensable to harden low C-Mn-Cr steel to a 0.2% proof stress of at least 100 kgf/mm.sup.2, leading to another problem that it is difficult to fabricate retaining rings of desired quality with such low C-Mn-Cr steel.